Re: question about rra
There are no stupid questions... well, I take that back. But this is not one of them. But it does show you need to learn to do some research prior to posting. Google has a great search engine and with generic searches like "6.5 Creedmore in ar15" can turn up lots of responses to your question.
In general SgtButt covered it. It's really not a RRA thing.. it's a.. putting a 308AR platform cartridge into a AR15 platform gun, which obviously can't be done. If you want to stay with the AR15 platform there are numerous wildcat cartridges you can go with but none will offer the same down range ballistics of a bigger cartridge. If just shooting paper, then some can be comparable for drop/wind but have no energy on target, which for paper is fine. If you are going for down range energy, you will have to look to the 308AR platform, which opens up even more research as not all of the companies that make these platforms parts are fully interchangeable like the AR15 platform makers. Some of the 308AR platform players are RRA (LAR8), Armalite (AR-10), DPMS (LR308), Larue, LMT, etc. When picking a platform rifle, look at your needs and budget and go from there. You don't need MagPul mags, they are a good product but I wouldn't and didn't base my purchase on what magazine goes in it. I would more look at the reputation of the company that made your rifle and their track record. I mean, how many mags do you really need even if they are $50 each.. 3, 4? Heck, cheap ass Glock mags are $25 each now.
My advice before moving forward would be to read to page 40 in this section, every thread related to 308AR's/AR-10,LAR8,MWS,OBR and 6.5Creedmore, .260remington, .308. Then you should have a good understanding of what's what. If you still are confused, read the next 60 pages of threads, it's all there.
Cheers, welcome and good luck.